1. Field of the Invention
The present invention relates to precipitated silicas having special surface qualities, to a process for preparing them and to their use for thickening sealants.
2. Discussion of the Background
Sealants are elastic substances that are applied in liquid to highly viscous form for the sealing of buildings or installations against water, atmospheric influence or aggressive media.
Silicone rubbers are compositions which are convertible into the elastomeric state and comprise as their base polymers polydiorganosiloxanes containing groups amenable to crosslinking reactions. Suitable such groups include, primarily H atoms, OH groups and vinyl groups, which may be located at the chain ends, or else may be incorporated in the chain. Incorporated into this system are fillers as reinforcers, their nature and amount significantly influencing the mechanical and chemical behavior of the vulcanizates. Silicone rubbers can be colored with inorganic pigments. One distinction is between high-temperature vulcanizing and room-temperature vulcanizing (HTV/RTV) silicone rubbers.
Among the room-temperature curing or vulcanizing silicone rubber compositions, it is possible to differentiate one-component (1K) and two-component (2K) systems. The first group (RTV-1K) polymerizes slowly at room temperature under the influence of atmospheric moisture, with crosslinking taking place through condensation of SiOH groups to form Si—O bonds. The SiOH groups are formed by hydrolysis of SiX groups of a species formed as an intermediate from a polymer with terminal OH groups and from what is called a crosslinker R—SiX3 (e.g. ═—O—CO—CH3, —NHR). In two-component rubbers (RTV-2K) the crosslinkers used are, for example, mixtures of silicic esters (e.g. ethyl silicate) and organotin compounds, the crosslinking reaction that takes place being the formation of an Si—O—Si bridge from ≡Si—OR and ≡Si—OH (-=methyl group; R=organic radical) by elimination of alcohol.
The thickeners used for RTV-1 K silicone rubber include silicas. In view of the sensitivity to hydrolysis of the silicone sealants, these silicas must introduce as little moisture as possible into the system. For this reason, fumed silicas have been used almost exclusively to date for this application. Hydrophilic silicas have not been used to date, on account of their high moisture content.
WO 2005/061384 shows the preparation and use—including use in silicone rubber—of silicas having a water absorption of <6% by weight and a DOP>300 ml/100 g. The silicas disclosed in the examples of WO 2005/061384, however, all have a water absorption of between 5.7% and 5.9% by weight and are therefore unsuitable for use in RTV-1K formulations. Accordingly, WO 2005/061384 describes only their use in silicone rubber formulations for extrusion processes (HTV).
EP 1557446 describes exclusively HTV silicone rubber formulations. The silicas employed therein have a loss on drying of <4% by weight. The formulations disclosed in EP 1557446 are used for producing insulating materials such as cable sheathing, for example.
In summary, therefore, it can be stated that the background art does not disclose any precipitated silicas which meet the exacting requirements for use in RTV-1K silicone rubber. There is therefore a strong need for precipitated silicas of this kind that are suitable for RTV-1K applications.